1. Field of the Invention
The present invention relates to a scanning optical device suitable for use in an image forming apparatus, such as a laser beam printer (LBP), a digital copier, or a multifunctional printer which performs an electrophotographic process. In particular, the present invention relates to a scanning optical device configured to be used in a tandem color image forming apparatus that can form a color image by guiding a plurality of light fluxes to a plurality of surfaces to be scanned.
2. Description of the Related Art
A known scanning optical device, such as an LBP, performs image recording by using a light deflector such as a rotating polygon mirror to periodically deflect light modulated according to image signals and emitted from a light source element. An imaging optical element, which can have f-θ characteristics, is adapted to allow the modulated light to converge on a surface of a photosensitive recording medium (photosensitive drum). The recording medium is thus optically scanned, thereby performing image recording.
FIG. 9 is a schematic diagram showing a substantial part of a known scanning optical device.
Referring to FIG. 9, light rays diverging from a light source element 1 are converted to substantially parallel light by a collimating lens 2, are limited by a diaphragm 3, and then enter a cylindrical lens 4, which can have predetermined refractive power, only in the sub scanning direction.
The parallel light rays that have entered the cylindrical lens 4 exit in parallel in a main scanning plane while, in a sub scanning plane, they converge to form a linear image on a deflecting surface (reflecting surface) 5a of a light deflector 5 such as a polygon mirror.
The light rays deflected from the deflecting surface 5a of the light deflector 5 are guided via an imaging optical element 6, which can have f-θ characteristics, onto the surface of a photosensitive drum 8 serving as a surface to be scanned. Rotating the light deflector 5 in a direction indicated by an arrow “A” allows optical scanning over the surface of the photosensitive drum 8 in a direction indicated by an arrow “B”, thereby recording image information.
As the use of color documents is becoming widespread in offices, image forming apparatuses, such as LBPs and digital copiers, are required to output color images at high speed. Under such an environment, a type of color image forming apparatus that becomes dominant, because of its high productivity, is a so-called tandem color image forming apparatus, in which a scanning optical device, a photosensitive member, and a developing unit are prepared for each color and arranged in parallel.
There are various types of known scanning optical devices configured to be used in such a tandem color image forming apparatus. For example, Japanese Patent Laid-Open No. 11-223783 discusses a color image forming apparatus in which a plurality of scanning optical devices corresponding to (four) respective colors are arranged in parallel. In addition, Japanese Patent No. 2725067 discusses an image forming apparatus in which a light deflector and imaging optical element (f-θ lenses) disposed in a light path between the light deflector and the surface of a photosensitive drum are shared among a plurality of light fluxes corresponding to (four) respective colors.
However, these scanning optical devices generally require several imaging optical elements (f-θ lenses) according to the number of colors. Moreover, since an incidence optical system disposed in a light path between a light deflector and a light source element cannot be easily shared among colors, a plurality of identical incidence optical systems can be prepared.
Therefore, scanning optical devices configured to be used in a tandem color image forming apparatus are more complex in structure and more expensive in cost, compared to the known single scanning optical device shown in FIG. 9.
Japanese Patent Laid-Open No. 2001-281575 discusses a scanning optical device in which an incidence optical system is shared among colors.
However, in this scanning optical device, the principal rays of four light fluxes corresponding to respective colors are spaced along a direction parallel to the rotational axis of the light deflector. This structure requires a large light deflector to allow the spatial separation of the four light fluxes on the deflecting surface.
In other words, since, in the sub scanning plane, the principal rays of the four light fluxes corresponding to the respective colors enter the deflecting surface of the light deflector at a right angle, the principal rays of the four light fluxes cannot be spatially separated on the deflecting surface without increasing the size of the light deflector.